Crude oil recovery by enriched gas drive



Aug. 23, 1966 CRUDE OIL PRODUCED OF PORE VOLUME OF THE SAND PACK) J. M. DUMORE 3,268,000

CRUDE OIL RECOVERY BY ENRIGHED GAS DRIVE Filed Aug. 14, 1963 O V l A I l l I A 1 1 2O 3O 4O 5O 6O 7O 8O 90 I00 PROPANE PRESENT IN DRIVE FLUID (MOLE PROPANE) INVENTORI JOHANNES M. DUMORE g fw HIS ATTORNEY United States Patent 3,268,000 CRUDE OIL RECOVERY BY ENRICHED GAS DRIVE Johannes M. Dumor, Rijswijk, Netherlands, assiguor to Shell Oil Company, New York, N.Y., a corporation of Delaware Filed Aug. 14, 1963, Ser. No. 302,103 6 Claims. (Cl. 1669) This invention relates to a method of recovering crude oil from a subsurface reservoir, which method is of the type generally referred to as miscible drive. In particular the present invention is concerned with drive fluids consisting of methane or an intermediate hydrocarbon or of mixtures of these compounds. By intermediate hydrocarbon is meant a paraffinic hydrocarbon having more than one but not more than six carbon atoms per molecule, or a mixture of such hydrocarbons.

In a miscible drive, the crude oil present in the pore space of the subsurface reservoir is recovered by displacing the oil by means of a drive fluid which is miscible with the crude oil. If the drive fluid is not miscible in all proportions with the crude oil, capillary forces at the interface between the drive fluid and the crude oil cause the retention of residual crude oil in the pore space behind the moving interface. If, however, the drive fluid is miscible in all proportions with the crude oil there are no capillary forces; no residual oil is left behind and consequently high recovery of the crude oil from the reservoir is possible.

The above-mentioned drive fluids are mostly less dense and less viscous than the crude oil which is to be displaced from the reservoir. Owing to their low viscosity, drive fluids have a tendency to finger into the crude oil, while owing to their low density, the acceleration of gravity tends to keep these drive fluids above the crude oil. Therefore, if the displacement is carried out in a downward direction in a dipping reservoir, the viscous fingering will be counteracted by the effect of gravity. Stable displacement, i.e., without fingering, will be obtained if the superficial rate of the fluids in the permeable medium is lower than the so-called critical rate.

By superficial rate of a fluid in a permeable medium is meant the volume of fluid passing through a unit area of a cross-section of the permeable medium perpendicular to the direction of flow of the fluid, per unit of time. The critical rate U can be calculated from the following formula:

ro rd (centimeters/second) in which:

.1 is the density of the crude oil in the reservoir (gram/ cubic centimeter),

p is the density of the drive fluid in the reservoir (gram/ cubic centimeter),

k is the permeability of the reservoir (square centimeter),

11. is the viscosity of the crude oil in the reservoir (poise),

,u is the viscosity of the drive fluid in the reservoir k is the relative permeability of the reservoir to the crude oil to be recovered (dimensionless),

k is the relative permeability of the reservoir to the drive fluid (dimensionless),

g is the acceleration of gravity (cent-imeters/second and a is the angle between the horizontal and the direction of flow of the drive through the reservoir, in which the crude oil is being displaced in a downward direction.

Tests have been conducted to illustrate the recovery of crude oil from a dipping permeable medium. For example, tests were carried out in a reservoir or a test-tube filled with grains (e.g. sand) by means of displacement in a downward direction by using a drive fluid consisting of methane or of an intermediate hydrocarbon or of mixtures of these components. These tests revealed that recoveries at, for example breakthrough or at a cumulative injection of 1 pore volume of the drive fluid, are higher at a superficial rate of the fluids lower than the critical rate U in the perm-cable medium, than at a superficial rate of the fluids in the permeable medium higher than said critical rate U. This result could be expected in view of the stability of the displacement.

Furthermore, it has been found that at a superficial rate of the fluids in the permeable medium lower than the critical rate U, recoveries are higher when using drive fluids which are miscible in all proportions with the crude oil than when using others which are not. This result could be expected in view of the effect of capillary forces.

These findings suggest that one should refrain from using superficial rates of the fluids in the permeable medium higher than said critical rate U and from using methane as a component in the drive fluid, since the miscibility of methane with crude oil is considerably lower than that of intermediate hydrocarbons with crude oil under equivalent conditions.

Unfortunately, however, superficial rates of the fluids in the permeable reservoir lower than the critical rate U correspond to crude oil production rates which for some fields are too low to be economical. In such cases, it might be expected that when employing superficial rates higher than the critical rate U, of the fluids in the permeable reservoir the result would be a decrease in the recoveries of crude oil at, say, breakthrough or at a cumulative injection of 1 pore volume of the drive fluid. Furthermore, when using these higher rates it might be expected that the best recoveries will be obtained if methane has not been used as a component in the drive fluid, because of the low miscibility of methane with the crude oil to be recovered.

Contrary to expectations, however, it has now been found that there are drive mixtures consisting substantially of an intermediate hydrocarbon and methane, by means of which at superficial rates higher than the critical rate for methane as drive fluid under the same conditions, but not more than three times this critical rate, recoveries can be obtained at, say, breakthrough or at a cumulative injection of 1 pore volume of the drive fluid, that are higher than the recoveries which would be obtained at the same rate using a drive fluid in which methane has not been used as a component.

It is, therefore, an object of the present invention to provide a method to facilitate the recovery of crude oil from a subsurface reservoir through the use of a drive fluid comprising an intermediate hydrocarbon and methane mixture, which recovery is accomplished at a super ficial drive fluid rate exceeding the critical superficial rate for methane under the same conditions and is higher than that obtainable at the same superficial rate using a drive fluid in which methane is not a component.

The foregoing and other objects of the invention will be apparent from the following description and accompanying drawing, the single figure of which represents the results of experiments to determine the recoveries possible with various compositions of drive fluids.

According to the invention, a method of recovering crude oil from a subsurface reservoir provided with at least one injection well and at least one production well comprises the following steps:

(a) injecting a selected quantity of a drive fluid consisting substantially of a selected ratio of methane and an intermediate hydrocarbon into a test tube containing a permeable medium containing a fluid having substantially the same characteristics as the crude oil present in the subsurface reservoir and under conditions of pressure and temperature substantially equal to the conditions to which the crude oil is subject in the subsurface reservoir, and measuring the amount of fluid displaced by the drive fluid from the test tube, in which the ratio between the superficial rate of the fluids in the permeable medium and the critical rate as determined by the formula:

(centimeters/second) is between 1 and 3, where:

[If is the density of the fluid to be recovered gram/ cubic centimeter),

p is the density of the methane (gram/ cubic centimeter),

k is the permeability of the medium in the test tube (square centimeter),

is the viscosity of the fluid to be recovered (poise),

a is the viscosity of the methane (poise),

k is the relative permeability of the medium in the test tube to the fluid to be recovered (dimensionless),

k is the relative permeability of the medium in the test tube to the methane (dimensionless),

g is the acceleration of gravity (centimeters/second and a is the angle between the horizontal and the direction of flow of the drive through the permeable medium contained in the test tube in which the fluid is being displaced in a downward direction;

(b) repeating the experiment under substantially the same conditions with other selected ratios of methane and an intermediate hydrocarbon and measuring in each experiment the amount of fluid displaced from the permeable medium contained in the test tube;

(c) injecting into the injection well a drive fluid consisting substantially of methane and an intermediate hydrocarbon, the composition of the drive fluid being substantially the same as that of the drive fluid with which in the experiments the highest recovery was obtained at, say, breakthrough or at a cumulative injection of 1 pore volume of the drive fluid, and the ratio between the mean superficial rate of the fluids in the reservoir and the critical rate, as determined by the formula:

being equal to the ratio at which the experiments were carried out, in which formula:

(centimeters/second) p ,is the density of the crude oil (gram/cubic centimeter),

p is the density of the methane (gram/cubic centimeter),

k is the permeability of the reservoir (square centimeter),

,u is the viscosity of the crude oil (poise),

,u is the viscosity of the methane (poise),

k is the relative permeability of the reservoir to the crude oil to be recovered (dimensionless),

k is the relative permeability of the reservoir to the methane (dimensionless),

g is the acceleration of gravity (centimeters/second and a is the angle between the horizontal and the direction of flow of the drive through the reservoir in which the crude oil is being displayed in a downward direction; and

(d) withdrawing crude oil originally contained within the subsurface reservoir via the production well.

The mean superficial rate of the fluids in the reservoir of the fluids occurring in the reservoir to be traversed by the drive fluid. Mathematically this can be expressed as:

1 V uf udV where:

E is the mean superficial rate V is the bulk volume of the permeable reservoir which has to be traversed by the drive fluid u is the superficial rate, which is a function of the space in the reservoir. This rate can be calculated from reservoir studies.

As intermediate hydrocarbons suitable for use as components of the drive fluid are more expensive than methane, the total amount of intermediate hydrocarbons to be used has to be restricted, so as to reduce costs. Preferably, the total amount of intermediate hydrocarbons to be used as components of the drive fluid takes up, under mean reservoir pressure and temperature conditions, a volume between 3 and 10 percent of the hydrocarbon pore volume of the reservoir to be traversed by the drive fluid. After the drive fluid has been injected into the reservoir, injection may be continued with a scavenging fluid until the production rate of the crude oil becomes too small for economic production. Lean gas (i.e. a gas which consists substantially of methane) may be used as a scavenging fluid.

The ratio between the volume of the intermediate hydrocarbons to be used as components of the drive fluid and the volume of the hydrocarbon pore space of the reservoir to be traversed by the drive fluid substantially equals the ratio between the volume of the intermediate hydrocarbons used as components of the drive fluid used in the experiments and the volume of the pore space of the ermeable medium contained in the test tube. If in the experiments a scavenging fluid has to be used, it ought to have substantially the same composition as the scavenging fluid which will be used in the reservoir. It has been found preferable to choose propane as the intermediate hydrocarbon component of the drive fluid.

To illustrate the present invention by way of example, a description follows of the experiments carried out to ascertain the composition of a drive fluid consisting substantially of methane and propane. By means of these experiments, the composition of drive fluid resulting in the highest recovery of crude oil at, for example, breakthrough or at a cumulative injection of 1 pore volume of the drive and scavenging fluids, could be determined for a particular subsurface reservoir in the State of Brunei (British Borneo).

From the studies of this particular oil field, the subsurface reservoir was found to consist of a rather uniform consolidated sand layer having a mean permeability of 10 cm. and a sin a, of 0.2. Mean reservoir pressure and temperature conditions proved to be 100 atmospheres absolute and degrees eentigrade. Under these conditions the crude oil present in the reservoir was found to have a density of 0.73 gram/cm. and a viscosity of 0.0055 poise, while under the same conditions methane has a density of 0.06 gram/cm. and a viscosity of 0.00014 poise. The relative permeability of the subsurface reservoir to the crude oil and .to methane proved to be nearly 1 and on average 0.2, respectively. The subsurface reservoir was provided with one injection well at the crest and one production well at the down dip side.

The injection and production rates to be applied in this particular subsurface reservoir correspond to a mean superficial rate of the fluids in said reservoir of about 5.4 l0- cm./second.

The total volume of propane available for injection into the reservoir, measured under mean reservoir pressure and temperature conditions, was about 6 percent of the hydrocarbon pore volume of said reservoir. After the drive fluid had been injected, methane was to be used as a scavenging fluid.

The critical rate for a methane drive in the relevant reservoir can be calculated, using the abovementioned data, from the formula:

(centimeters/second) where pc is the density of the crude oil gram/ cubic centimeter),

p is the density of methane g-ram/ cubic centimeter),

k is the permeability of the reservoir (square centimeter),

,u is the viscosity of the crude oil (poise),

pm is the viscosity of methane (poise),

k is the relative permeability of the reservoir to the crude oil to be recovered (dimensionless),

k is the relative permeability of the reservoir to methane (dimensionless),

g is the acceleration of gravity (centimeters/'second and o is the angle between the horizontal and the direction of flow of the drive through the reservoir in which the crude oil is being displaced in a downward direction.

The critical rate is approximately 2.7 1O- centimeters/ second. Hence the ratio between the required mean superficial rate of the fluids in the reservoir (5.4 centimeters/ second) and the calculated critical rate when using methane as a drive fluid (2.7 l() centimeters/ second) is 2. The same ratio has to be taken in the experiments to ensure that the degree of instability, ie the degree of viscous fingering, at which the crude oil will be displaced by the drive fluid is the same both in the reservoir and in the experiments.

Just as in the reservoir, the total volume of propane available for injection in each experiment was selected at that amount of propane which, when measured at 100 atmospheres absolute and 75 C., was equal to 6 percent of the hydrocarbon pore volume of the sand pack used in the experiments. Furthermore, where :a scavenging fluid had to be employed in an experiment, methane was used.

The experiments were carried out in a vertically arranged sand-packed stainless steel tube having a length of 100 centimeters and an internal diameter of 6.25 centimeters. The permeability of the sand pack was about 7 1() cm. The tube was provided at its top end with an inlet for the drive fluid and the scavenging fluid, if any, and at its bottom end with an outlet for withdrawing any fluids expelled from the sand pack by the fluids injected.

As the equipment required to mete-r and supply fluid to the sand pack, as well as to withdraw and meter fluid passing out of the sand pack, is well known in the art, it need not be described here in further detail. During the experiments, the whole system was kept at a temperature equal to the mean reservoir temperature of 75 C.

Before carrying out each experiment, the sand pack in the tube was cleaned by extracting it with gasoline and dried by evaporating the gasoline in a stream of nitrogen passed through the sand pack. Then the sand pack was saturated with a hydrocarbon fluid of substantially the same composition as the crude oil present in the subsurface reservoir, While the pressure was increased to the mean reservoir pressure of 100 atmospheres absolute.

Each drive was performed by injecting the drive fluid into the top end ofthe tube at a constant rate, while fluids were withdrawn from the outlet of the tube at such a rate that the pressure at the top end of the tube remained 100 atmospheres absolute. If the volume of the drive fluid was less than one pore volume of the sand pack in the test tube, the drive fluid was followed by methane as a scavenging fluid. The value of the constant injection rate had been chosen such that the ratio described earlier had the correct value which was found to be 2. The critical rate of a methane drive in the permeable medium of the test tube was calculated from the formula:

Each injection was continued until a cumulative volume of the fluids equal to one pore volume of the sand pack had been injected at the top end of the tube at atmospheres aboslute and 75 C. The amount of crude oil produced by the drive was measured at the moment that the drive fluid broke through, i.e. was Withdrawn from the outlet end of the sand-packed tube, and at the moment that a cumulative volume of fluids equal to one pore volume of the sand pack had been injected into the top end of the tube.

Various drives were carried out, each using a drive fluid whose composition differed from those of the other drives. The first experiment was carried out with a drive fluid consisting of propane only and having a volume equal to the amount of propane available for the experiments (-i.e. -6 percent of the hydrocarbon pore volume of the sand pack). The other experiments were carried out by means of drive fluids consisting of propane and increasing amounts of methane, the amount of propane being equal to the above-mentioned 6 percent of the pore volume, as long as the total amount of the drive fluid (measured at 100 atmospheres absolute and 75 C.) did not exceed a volume equal to one pore volume of the same pack. If the volume of the drive fluid exceeded one pore volume of the sand pack, a smaller amount of propane was mixed with methane up to one pore volume of the sand pack.

In the accompanying drawing the results of the experiments described above are graphically represented. Along the abscissa is plotted the amount of propane present in the drive fluid in mole percent and along the ordinate the amount of crude oil produced as a percentage of the pore volume of the sand pack. The lower curve in the graph indicates the relationship between the amount of oil produced and the composition of the drive fluid at the moment of breakthrough of the drive fluid. The upper cunve indicates the same relationship, but at the moment that a total volume of drive fluid and scavenging fluid equal to one pore volume of the sand pack has been injected into the same.

As will be clear from the graph, optimum recovery results were obtained under the described conditions by using a drive fluid consisting of 60 mole percent of propane and 40 mole percent of methane.

The drive fluid was injected into the injection well at the crest of the of the subsurface reservoir at such a rate that the mean superficial velocity of the fluids in the reservoir was about 5.4 10 centimeters/second. Crude oil displaced by the drive fluid was recovered through the production Well located at the down dip side of the reservoir.

To summarize, the present invention provides a method for recovering a maximum amount of crude oil from a formation with a drive fluid comprising a methane/intermediate hydrocarbon mixture. The drive is aifected in a restricted velocity range in which displacement in the formation is unstable and with a drive mixture having a particular composition experimentally shown to produce optimum crude oil recovery. The experiments used to determine the drive mixture composition are conducted under conditions simulating those of the formation in pressure and temperature, as well as structure. It is noted that latter characteristic distinguishes the experimental technique -from phase equalibria techniques wherein pressure and temperature simulations are used only to determine the miscibility of fluids. When comparing the experimental technique of the present invention to techniques only capable of determining miscibility, it can be centimeters/ second) seen that the displacement characteristics of a drive fluid are greatly affected by formation structure as well as formation temperature and pressure.

In conclusion, attention is directed to the fact that with the present invention, the slug size of an intermediate hydrocarbon drive fluid may be increased by the addition of methane. Thus, the invention provides a method of establishing high crude oil recovery with an intermediate hydrocarbon drive fluid while decreasing the intermediate hydrocarbon necessary for a drive slu g. The latter feature is particularly desirable where available intermediate hydrocarbons are fairly limited.

I claim as my invention:

1. A method of recovering crude oil from a subsurface reservoir providing with at least one injection Well and at least one production well, comprising:

(a) conducting an experiment by injecting a selected quantity of a drive fluid consisting substantially of a selected ratio of methane and an intermediate hydrocarbon into a test tube containing a permeable medium containing a fluid having substantially the same characteristics as the crude oil present in the subsurface reservoir and under conditions of pressure and temperature substantially equal to the conditions to which the crude oil is subject in the subsurface reservoir, and measuring the amount of fluid displaced by the drive fluid from the test tube, in which the ratio between the superficial rate of the fluids in the permeable medium and the critical rate, as determined by the formula:

(centimeters/second) fi i mf mm is between 1 and 3, where:

pf is the density of the fluid to be recovered (gram/cubic centimeter),

p is the density of the methane (gram/cubic centimeter),

k is the permeability of the medium contained in the test tube (square centimeter),

a is the viscosity of the fluid to be recovered (poise),

a is the viscosity of the methane (poise),

k is the relative permeability of the medium in the test tube to the fluid to be recovered (dimensionless),

k is the relative permeability of the medium in the test tube to the methane (dimensionless),

g is the acceleration of gravity (centimeters/second and a is the angle between the horizontal and the direction of flow of the drive through the permeable medium contained in the test tube in which the fluid is being displaced in a downward direction;

( b) repeating the experiment under substantially the same conditions with other selected ratios of methane and an intermediate hydrocarbon and measuring in each experiment the amount of fluid displaced from the permeable medium in the test tube;

(c) injecting into the injection Well a drive fluid consisting substantially of methane and an intermediate hydrocarbon, the composition of the drive fluid being substantially the same as that of the drive fluid with which in the experiments the highest recovery was obtained at a cumulative injection of at least the volume of the drive fluid necessary for breakthrough and the ratio between the mean superficial rate of the fluids in the reservoir and the critical rate, as determined by the formula:

being substantially equal to the ratio at which the experiments were carried out, in which formula:

pc is the density of the crude oil (gram/ cubic centimeter),

p is the density of the methane (gram/ cubic centimeter),

k is the permeability of the reservoir (square centimeters),

pc is the viscosity of the crude oil (poise),

p is the viscosity of the methane (poise),

k is the relative permeability of the reservoir to the crude oil to be recovered (dimensionless),

k is the relative permeability of the reservoir to the methane (dimensionless),

g is the acceleration of gravity (centimeters/second and a, is the angle between the horizontal and the direction flow of the drive through the reservoir and in which the crude oil is being displaced in a downward direction; and,

(centimeters/second) (d) withdrawing crude oil originally contained within the subsurface reservoir via the production well.

2. A method as claimed in claim 1, wherein the ratio between the volume of the intermediate hydrocarbon used as a component of the drive fluid and the volume of the hydrocarbon pore space of the reservoir to be traversed by the drive fluid substantially equals the ratio between the volume of the intermediate hydrocarbon employed as a component of the drive fluid used in the experiment and the volume of the pore space of the permeable medium contained in the test tube.

3. A method as claimed in claim 2, wherein the total volume of the intermediate hydrocarbon used as a component of the drive fluid occupies, under mean reservoir pressure and temperature conditions, between 3 and 10 percent of the hydrocarbon pore volume to be traversed by the drive fluid.

4. A method as claimed in claim 1, wherein a scavenging fluid is injected after the drive fluid.

5. A method as claimed in claim 4, wherein methane is used as a scavenging fluid.

6. A method as claimed in claim 1 wherein the intermediate hydrocarbon used as a component of the drive fluid is propane.

References Cited by the Examiner UNITED STATES PATENTS 3,035,637 5/1962 Allen l669 3,139,929 7/1964 Habermann l669 3,203,481 8/1965 Dew et al. l669 OTHER REFERENCES Kieschnick, In, What is Miscible Displacement, The Petroleum Engineer, August 1959, pp. B-56, 66, 70, 77, 80, 84, 96 and 98 relied on.

Slobod et al.: Use of a Graded Viscosity Zone to Reduce Fingering in Miscible Phase Displacements, Producers Monthly, August 1960, pp. 12, 14, 15, 16, 18 and 19.

CHARLES E. OCONNELL, Primary Examiner.

S. J. NOVOSA-D, Assistant Examiner. 

1. A METHOD OF RECOVERING CRUDE OIL FROM A SUBSURFACE RESERVOIR PROVIDING WITH AT LEAST ONE INJECTION WELL AND AT LEAST ONE PRODUCTION WELL, COMPRISING: (A) CONDUCTING AN EXPERIMENT BY INJECTING A SELECTED QUANTITY OF A DRIVE FLUID CONSISTING SUBSTANTIALLY OF A SELECTED RATIO OF METHANE AND AN INTERMEDIATE HY DROCARBON INTO A TEST TUBE CONTAINING A PERMEABLE MEDIUM CONTAINING A FLUID HAVING SUBSTANTIALLY THE SAME CHARACTERISTICS AS THE CRUDE OIL PRESENT IN THE SUBSURFACE RESROIR AND UNDER CONDITIONS OF PRESSURFACE TEMPERATURE SUBSTANTIALLY EQUAL TO THE CONDITIONS TO WJSICH THE CRUDE OIL IS SUBJECT IN THE SUBSURFACE RESERVOIR, AND MEASURING THE AMOUNT OF FLUID DISPLACED BY THE DRIVE FLUID FROM THE TEST TUBE, IN WHICH THE ATIO BETWEEN THE SUPERFICIAL RATE OF THE FLUIDS IN THE PREMEABLE MEDIUM ANND THE CRITICAL RATE, AS DETERMINED BY THE FORMULA: 